QFX5100 FAN may incorrectly report failure by Junos because the unit spins at RPM that outside the threshold defines in Junos. These fans are not failing and there is no need to replace them.
In Junos CLI, QFX5100 FAN may report failure while the fan is spinning at certain RPM.
root> show chassis environment
Class Item Status Measurement
...
Fans FPC 0 Fan Tray 0 Failed
...
root> show chassis fan
Item Status RPM Measurement
FPC 0 Tray 0 Fan 0 OK 10305 Failure
FPC 0 Tray 0 Fan 1 OK 9642 Failure
...
Fans are spinning at RPM which is outside the tolerated threshold defined by Junos.
The new Junos will extend the tolerance for allowed RPM from +-20% to +30%/-20%